In any non-nuclear power station or ship operating on the Brayton Cycle, the cost of the steam boilers is one of the major capital costs. Shipping companies which used to operate on the Brayton cycle in their ships have switched to the Otto or Diesel cycle because they can operate at 50-54% efficiency. Conventional boilers occupy considerable real estate and are involved in most of the legislation regarding permitting for power stations. They have some inherent disadvantages, which are taken for granted in the power industry. Because the boiler tubes holding steam can only accept a relatively small amount of heat per unit area, the area of boiler tubes in a large power station is quite considerable. The heat losses through the outside of the boiler itself are a major problem in design and operation. There is a significant volume of heat carried away in the ash, but by far the major heat loss from the boiler is up the chimney. Even with economisers, which are essentially a secondary boiler used to recover the low-grade heat after the main boiler, the flue gases are still quite hot, and represent the most significant loss of energy in the whole boiler system.
Steam moderated combustion is a technique that has been employed for over one hundred years in steam engines and steam powered generators. The exhaust steam from the pistons driving the wheels in a classical steam engine is vented up the exhaust stack of the steam engine primarily to assist in the draft through the combustion chamber but it has the added benefit of being catalyzed by the carbon particles present in the exhaust and breaking down into hydrogen and carbon monoxide which is then combusted. This results in much lower exhaust emissions from the steam engine, and a significant increase in the draft in the boiler and hence the rate of combustion. The reduction in soot is a feature which is only recently appreciated. Many boilers running on dirty fuel use either continuous or pulsed steam injection to clean the fire tubes. This steam injection keeps the interior of the boiler clean but also assists in the catalytic breakdown of large soot particles which would otherwise be ejected from the exhaust stack.
Modern steam moderated combustion has two principally different aims. One is to increase the efficiency of combustion of problematic fuels. The other relates to oxygen (instead of air)-assisted combustion to generate steam and carbon dioxide without the nitrogen diluent associated with air combustion. The carbon dioxide so produced is not diluted with nitrogen and is readily separated using a variety of commonly available methods. The carbon dioxide so produced is then often sequestered in the ground.
There are a number of flare gas systems that are designed primarily to reduce or ameliorate the black smoke generated by flares. The auto ignition temperature of most hydrocarbon fuels is between about 300° C. and about 650° C. and it is not commonly realized in the industry that, if the steam is above 700° C., even if the fuel air ratios are not correct, ignition and combustion will proceed without an external ignition source.
Steam or water injection has been commonly employed since the advent of internal combustion engines. Modern heavy fuel ship engines use water or steam injection at different phases of the typical two stoke cycle to once again aid in the clean combustion of low rank fuels and increase the overall thermodynamic efficiency.
Electrically assisted combustion systems involve an exchange of electrons between the fuel and the oxygen in the air. This simple fact is quite often ignored in the fundamental design of combustion systems. Many combustion engineers know that something like this is necessary for stable combustion and become experts in designing flame front plates or other devices which are not commonly realized as electron donors. There are a number of companies that offer electrically assisted combustion. These systems are used when for whatever reason the combustion is not stable. With the addition of appropriate voltages amperages and in the right position, a given burner flame can be moderated or controlled across a very wide fuel/air ratio at a wide variety of pressures. If a given burner is driven too hard, then there is invariably a very significant Joule Thompson cooling effect across the burner. This quite often results in the burner flame being extinguished because the fuel and air mix is reduced below the auto ignition temperature. By injecting electricity into the burner at the appropriate place a given burner can be stabilized and run efficiently on inconsistent fuel rates and fuel/air ratios. With the injection of electricity, the effective auto ignition temperature may be dropped by about 2° C. to 300° C. Electrical assistance in the combustor or burner may lead to very high turndown ratios of better than 10-to-1.
A number of companies around the world have patented and developed ultrasonically assisted combustion burners with a view to be able to efficiently combust low rank coals and shale oil directly. These ultrasonic combustors effectively fragment the fuel and mix it thoroughly with the air giving more efficient fuel utilization and more complete combustion. Ultrasonic burners are typically much smaller than conventional burners of the same output because of the fuel fragmentation and a very thorough mixing in a very short flight path and hence a much smaller burner. Ultrasonic burners also enable the efficient combustion of oil and water emulsions which under normal circumstances would be nonflammable. Ultrasonic burners are also used to thoroughly mix limestone and other Sulphur scavengers with high Sulphur fuel which reduces the need for post-combustion gas cleanup.
Steam eductors have been around for over 100 years and at first glance the principals involved seem quite counterintuitive. Water typically is pumped into the boiler in boiler systems to replace the water lost into the steam which is used in whatever application is needed. The modern approach simply is to a use high-pressure water pump. However, in the past a steam eductor was commonly used to pump freshwater back into the boiler using the steam pressure in the boiler. This can be considered counterintuitive because one may wonder how the steam pressure in the boiler can be used to pump water back into the boiler. With the use of a steam educator, which comprises one or more venturies in series, the water is accelerated up to the point where its inertia is high enough to overcome pressure inside the boiler. In essence, one is virtually lifting oneself by pulling on one's bootstraps. This same principle can be used to compress air. With the use of suitable venturi, steam may be used to entrain air and thereby compress it typically up to half the pressure of the incoming steam.
For example, if the steam pressure is 60 bar or 1000 psi, then it may be used to compress an equivalent weight of air of about 30 bar or 500 psi. The actual pressures delivered depend on a number of factors but this is presented here solely as a general rule.